Most vertebrate genes are split into multiple exons and introns; introns are large; exons, in contrast, are very small. The average size of a vertebrate internal exon is 137 nucleotides; very few exceed 300 nucleotides. Occasionally, large exons exist. This proposal is concerned with investigating if the splicing machinery restricts exon size; if it does, how do large exons by-pass the normal restrictions on size during splicing; and is there a functional advantage to large exons? One naturally occurring large exon is exon 26 of the murine and human apolipoprotein-b gene. This exon is over 7 kb in length. In human liver it is spliced correctly to produce the large apo-bl00 protein of 512 kd involved in the metabolism of endogenous lipids. In human intestine, a mRNA is produced that includes an RNA editing event. Editing produces a translational stop codon within exon 26; translation produces a short apo-b48 protein of 252 kd involved in the metabolism of exogenous lipids. Editing is accompanied by polyadenylation within exon 26. This proposal describes experiments designed to understand how exon 26 is correctly recognized in liver and alternatively recognized in intestine. The regulation of apolipoprotein-b is important to normal lipid metabolism and human heart disease. Understanding the mechanism whereby the two forms of apo-b are generated should increase our knowledge of heart disease and how to combat it.